Automobile bodies and many industrial and consumer articles are spray painted in enclosed areas called paint spray booths. The purposes of these booths are to contain any fumes or paint overspray, reduce the risk of contamination of the part being painted and protect the health of the workers at the site. Smaller booths may employ filters to remove the paint overspray from the air but larger systems usually use a recirculating water system for this purpose. In operation, the units to be painted generally pass through the work area of the booth. Air flow through the booth forces the paint overspray into intimate contact with the recirculating water, thereby effectively scrubbing the paint solids from the air into the water. This may occur by forcing the air through a water curtain, some type of water spray or a venturi scrubber. Irrespective of how it is done, the net effect is that oversprayed paint contacts and is captured by the recirculating water of the spray booth.
The amount of oversprayed paint contacting recirculating water in a paint spray booth may change depending on a number of variables, including plant or process shutdowns, the size and shape of the object being painted, the type of spray equipment used, the spraying and purge technique used, the water flow rate and the type of paint used. Even though improved coating methods have significantly reduced the amount of paint overspray, up to about 50% of the total paint sprayed may not reach the article being painted. As a result, significant concentrations of paint can build up in the system. Since the majority of paints used today must be baked at elevated temperatures in order to cure properly, this paint can remain tacky indefinitely. It may coalesce into a mass of sticky material that can plug pipes, fans and recirculation pumps. In this way it can significantly reduce scrubbing efficiency in the booth. This can lead to an air imbalance which would not only result in a poor paint finish but also allow hazardous paint emissions to be discharged into the air. These conditions may also result in serious safety hazards to paint spray booth personnel. Also, federal regulations now limit the amount of volatile organic compounds (i.e., vocs) that can be released at a given plant site. Organic solvent diluents used in solvent-based paint are a major source of vocs.
Other problems may also develop. For example, the tacky paint deposits are excellent food sources for bacteria and fungus growths. These in turn can cause odor problems and spot corrosion in the system. The paint solids left in the water remain sticky and can cause expensive separation and disposal problems.
These problems demonstrate the desirability of effectively treating paint spray booth water systems so as to minimize the deposition of sticky, oversprayed paint on critical operating parts of the system; to make the resulting sludge non-tacky and easy to remove; and to provide water of sufficient quality that it can be recirculated in the system.
As paint technology has advanced, so has the quality of chemical treatments used to detackify the paint and provide a manageable sludge. There are presently a large variety of programs available for this purpose. For example, the use of water soluble polymers in conjunction with amphoteric metal salts which form insoluble metal hydroxides at pH's&gt;7 represents one such treatment. The use of combinations of this type are described in the following U.S. Pat. Nos. 3,861,887 to Forney; 3,990,986 to Gabel et al; 4,002,490 to Michalski et al; 4,130,674 to Roberts et al; and 4,440,647 to Puchalski. Further, U.S. Pat. No. 4,637,824 to Pominville discloses the use of silicates and polydiallyldialkyl ammonium halides with amphoteric metal salts, and U.S. Pat. No. 4,853,132 to Merrell et al discloses the use of precipitates formed by the reaction of cationic polymers and salts of inorganic anions to detackify solvent based paints. Bentonite clays, aluminum salts and zinc salts have also been used with cationic polymers.
U.S. Pat. No. 4,656,059 to Mizuno et al relates to the use of melamine-aldehyde acid colloid solutions for treating paint in wet spray booths and U.S. Pat. No. 4,629,149 to Leitz et al relates to the use of urea or amino triazine-aldehyde condensation reaction products in combination with water swellable clays to treat paint spray booth wastes. EP 0293129 discloses the use of melamine formaldehyde-type acid colloids, in conjunction with an alkalinity source, to treat oversprayed solvent based paints in paint spray booths, and U.S. Pat. No. 4,935,149 discloses the use of formaldehyde scavenged melamine-formaldehyde polymers to treat paint spray booth waters. Copending application U.S. Ser. No. 588,997 relates to the use of melamine formaldehyde acid colloids in paint spray booths containing both water and solvent based paints.
None of these references, however, disclose or suggest the use of a chelating or sequestering agent to adjust the hardness of the recirculating water of a paint spray booth. The instant inventor has found the addition of such compounds to substantially improve the performance of paint spray booth treatment methods, particularly acid colloid-based methods, by increasing the dispersibility of solvent borne paints captured by paint spray booth waters.